1. Field of the Invention
The present invention relates generally to optical sources and, in particular, to controlling an optical source during start up.
2. Brief Description of Related Developments
Commercial wavelength division multiplex (WDM) systems, especially of the “dense” type (DWDM) provide high transmission capacity operating with channel spacings of 50-100 GHz.
In order to ensure the wavelength stability required for the optical source, real time control of the wavelength emitted is an essential feature to be provided (preferably together with automatic power and extinction ratio control) in any of the transmitter modules included in a WDM/DWDM arrangement.
The modules in question generally include a laser diode as the optical source emitting signal light together with a so-called “wavelength locker” arrangement—including a wavelength selective optical component and photodiodes to detect any wavelength and power variations in the laser source, a laser driver to bias the laser diode and a temperature conditioning element such as a thermoelectric cooler (TEC)—typically a Peltier element—for controlling the temperature of the laser diode together with its drive circuit.
One of the most critical phases of operation of such a module is the start-up phase.
During the start-up phase, the optical source is turned on and, within the first few hundreds microseconds, its temperature may significantly change. The wavelength of the radiation emitted may consequently exceed the maximum allowed fluctuations, while the wavelength and power control systems usually associated with such a module (and intended essentially to ensure source stability during normal operation) may operate with different time constants during the start-up phase thus undesirably contributing to such fluctuations.
In fact wavelength stability of the optical source must be ensured also under these conditions, in order to avoid that undesired large fluctuations in the output wavelength of the radiation emitted by the module being turned-on may adversely affect performance in the adjacent channels.
Prior art arrangements providing for optical wavelength and/or power control using analogue techniques, such as disclosed e.g. in U.S. Pat. No. 5,825,792 are not in a position to properly ensure such a strict wavelength control during the start-up phase. The same also applies to arrangements as disclosed e.g. in U.S. Pat. No. 5,438,579 providing for fairly sophisticated wavelength stabilization against temperature-related variations.
Certain DWDM modules are also known in the art including optical switches or controlled attenuators to prevent light emission when the laser diode is turned on, so that the laser beam is emitted only after the undesired wavelength variations are dispensed with. The main drawback of these arrangements lies in that additional elements (the optical switch or controlled attenuator) must be included in the module. Also, these additional devices may undesirably interfere with laser emission during normal operation.